1. Field of the Invention
This invention relates generally to optical catheter systems used for measuring properties of materials in suspension in fluids. It relates more particularly to diagnostic cardiovascular catheters which are used for short-term diagnoses, and which include optic fibers--whose polished tips are immersed in a patient's bloodstream --for reflectometric assays of blood characteristics.
For such measurements the tips of the optic fibers are usually positioned in the patient's right ventricle or pulmonary artery. At those locations nearly all the blood circulating in the body is collected and well mixed for return to the lungs. Consequently the conditions (such as oxygen-saturation level) there represent by definition a good average of venous conditions for the whole body.
By "short term" we mean brief periods such as minutes or hours. Much longer usages, such as months or years, characterize a very different type of device--a so-called "permanent" implant, often part of a pacemaker system.
2. Prior Art
Our invention has application in a great variety of fields. We are familiar with relevant prior art, however, only in the area of cardiovascular diagnostic catheters.
Cardiovascular catheters that include optic fibers are well known for short-term measurement of blood-oxygen saturation. Notable U.S. Pat. Nos. in this area are 4,295,470, 4,114,604 and 3,847,483 to Shaw et al., and 4,523,279 and 4,453,218 to Sperinde et al. These systems generally project light of two or three different wavelengths into the blood.
In general the blood differentially reflects the light components of different wavelengths, and the difference--or preferably the ratio--of reflectances varies with the level of oxygen saturation in the blood. Consequently the reflectance ratio can be used as a measure of oxygen saturation.
Such systems using just two wavelengths are subject to systematic measurement inaccuracies. As noted in the aforementioned Shaw U.S. Pat. No. 4,114,604, hematocrit (loosely speaking, the concentration of blood corpuscles in the blood) is among several factors that "introduce errors into the oxygen saturation measurements" made using two wavelengths.
Shaw resolves this and other uncontrolled influences by introducing and detecting light of a third wavelength: this technique provides enough information to correct the oxygen measurement for the unknown effects. Such a refinement is of course very useful, but relatively cumbersome in requiring three different light sources.
Moreover, at least in the form proposed by Shaw it only corrects for the effects of hematocrit without actually. providing a usable measurement of hematocrit itself. This limitation is regrettable since hematocrit does have independent diagnostic significance.
Another area of prior research that may be relevant to the present field is represented by the paper "Implantable Telemetry for Measurement of O.sub.2 Saturation," due to J. M. Schmitt, F. G. Mihm, J. Shott and J. D. Meindl. Their article appeared in IEEE Frontiers of Engineering and Computing in Health Care at page 703 (1984).
This paper of Schmitt et al. is in a somewhat different subfield from the present invention, as it relates to long-term measurement of oxygen saturation and teaches away from the use of optic fibers.
The Schmitt et al. system incorporates implantable transducers positioned directly at the measurement site, within the patient's body. There is no optic fiber; rather, any necessary light source(s) and detector(s) are exposed directly to the patient's bloodstream. The system does measure both oxygen concentration and hematocrit.
The paper also suggests that the ratio of intensities detected at two different source/detector distances might be used to determine hematocrit, independent of oxygen saturation. The paper discloses no particular configuration for doing so.
In principle, data can be extracted from the Schmitt system (and from the patient's body) by radio telemetry to diagnostic instruments in the laboratory. In view of certain stringent limitations, however, the only useful applications of the Schmitt system would appear to be in implanted control systems for pacemakers.
Specifically, the Schmitt system has the disadvantages of very extreme costliness and fragility. Production and assembly techniques for such devices, as described by Schmitt et al., are extremely awkward and difficult.
In order to attain reasonable operational stability and reliability, the Schmitt group found it necessary to assemble the various elements of their device (including an integrated-circuit controller) on a silicon wafer, and then to encapsulate it. As will readily be appreciated by those skilled in the art, such procedures are among the most demanding and expensive of all industrial techniques.
The finished Schmitt device is an elaborate wafer-mounted transducer array, hermetically sealed with solder connections and so forth. Each optical transducer made by the Schmitt approach accordingly runs into the hundreds of dollars, at the present (1986) monetary scale.
This level of cost might conceivably be acceptable for permanent implants, but in short-term diagnostic equipment it is essentially out of the question. For sterility reasons, workers skilled in the art of short-term diagnostic cardiovascular catheters generally concur that such catheters must be discarded after just one use!
As to such disposable cardiovascular catheters, the costs of materials must be kept in the range of pennies and dollars. Only by such restraint can the overall price of a finished catheter with all component subsystems be, say, between one and two hundred dollars at most.
The general approach of Schmitt has been followed in an even more remotely related type of instrument reported in the paper "A New Noninvasive Backscattering Oximeter," by T. M. Donahoe and R. L. Longini, published in the Proceedings of the Seventh Annual Conference of the IEEE/Engineering in Medicine and Biology Society, in volume 1 of 2, at page 144 (1985).
Donahoe and Longini do not discuss cardiovascular catheters--or, indeed, catheters of any type, or for that matter any other device that is immersed in blood, or cardiovascular expeditions by any technique. Their invention is, however, an optical device for determining blood-oxygen saturation--and a parameter that is analogous to hematocrit as well.
The Donahoe system is a reflectance oximeter that is applied to external surfaces of the patient's body. It includes a light source that, in use, is positioned directly against the patient's body tissue, so that light from the source passes directly into the tissue.
The system receives light that is backscattered by the tissue itself and also by whatever blood is present within the tissue. The device receives the scattered light at two separated detectors, both also positioned directly against the patient's body tissue--so that light passes directly from the tissue into the detectors.
The detectors, however, are at fixed, known distances from the source; and the differential in source/detector distance is used to measure so-called "tissue hematocrit": volume of blood corpuscles per unit volume of body tissue.
It would be very desirable to provide at practical levels of cost a short-term diagnostic cardiovascular catheter system, capable of both hematocrit-corrected oxygen-saturation measurements and independent hematocrit measurements within the immediate cardiac system.
The 1984 Schmitt system provides such measurements, but at prohibitive cost. The 1985 Donahoe system provides only analogous measurements, for the periphery of the body.
A much more recent paper of J. M. Schmitt, F. G. Mihm and J. D. Meindl, appearing in Annals of Biomedical Engineering, volume 14, page 35 (1986) describes a transducer system for mounting at the end of a cardiovascular catheter. We mention this paper here for completeness, although it does not appear to be prior art with respect to the present document.
This later Schmitt et al. transducer, except for its positioning on a cardiovascular catheter, is similar to that described by the related group of authors in 1984.
The comments made above regarding prohibitive cost levels of the earlier system are equally applicable to the more recent one. Even the newer Schmitt system, although interesting, fails to satisfy modern diagnostic needs for small, highly precise instrumentation.
In particular, the Schmitt group reports measurement variation of "about 19%" (root-mean-square deviation). That value is an order of magnitude too large for state-of-the-art medical practice.
Furthermore, Schmitt's optical-sensor package was six millimeters long, two millimeters wide and one millimeter thick. Although the authors believed it "possible" to reduce the width to about one millimeter, even with such a reduction the package would be much too large to fit on the tip of a cardiovascular catheter for use in human beings.